Background of the invention:
a) Field of the invention:
This invention relates to the containing composition of a pigment for the formation of the light absorbing layer formed in the array of a matrix surrounding the light emitting pixels of the display devices like plasma display panels. The present invention is relates with the black glass composition and preparation of the said glass materials and the paste for application.
The present invention relates to the composition & preparation of paste of a pigment used for the formation of light absorbing layer formed in the array of light emitting discharge cells in plasma display panel.
b) Prior art of the invention:
In recent years, attention has been drawn to flat panel displays especially plasma display panels (PDP) as a large size thin flat plate color display device. PDP has a structure wherein many cells (fine discharge spaces) are formed by barrier ribs so that wide horizontal and vertical view-angle can be achieved. Phosphors are provided on the surface of the respective cells, and an inert gas is filled in such cells. Discharge takes place between electrodes in the cells which generates vacuum ultra violet light, which in turn excites the phosphors in the ground state to emit visible light. Such PDP is a self emitting type flat display. It can be made to have a large size. Thus, it is one of the most prospective display devices. This type of conventional AC PDP is described in US Patent no. 5661500 by Shinoda et al.
In display area, between the two horizontal adjacent arrays of pixel forming bus electrode sets (conventionally black silver electrode over the wide transparent ITO electrode) lies a black stripe over the inter pixel zone to work as shutter to stop the unwanted light reaching the observer's eye. This technology is explained in the patent No. US5,952,782 by Nanto et al.
With the improvement in the display performance, parameters like black luminance of the panel play a vital role in achieving a better contrast ratio. In an actual display operation condition of fully dark room viewing is never achieved ideally and there remains always some visible light on the display front which reflects back to the observer. The black luminance from the pixels are contributing to the better contrast ratio but this does not benefit the observer as the reflected light the front panel mix with the light emitted by the display and make the color impure. Over a period of last 15 years the black luminance of the display has got reduced to O.lcd/sq mt from a value as high as 2 cd/sq mt. The dark room contrast ratio has also improved from a mere 100:1 to 1.00.000:1. Therefore, the black color rendering has improved much. Therefore, in
the presence of reflected light coming from the non discharge area, true color balance as achieved by the display technology is never realized by the observer.
In an open straight rib pixel structure, when viewing the display screen from the front, the phosphor layer is hidden by the absorbing film in the non-luminous areas.
In several prior works, researchers claimed to prepare suitable glass powders for black matrix application for displays. The patent document US 6,410,214 B1, US 6,387,576 B2, US 6,208,404 B1, and US 6,117,294 describe several methods of formation of the black matrix. The composition therein offers different shades of black and these black colors are also sometime very much susceptible to shade variation owing to the variation in the furnace atmosphere during manufacturing of the panel. In turn, instead of fully absorbing the ambient light, the so called black matrix reflects some light and color purity degrades.
Therefore, it is important to develop a light absorbing material, which when used can help overcome the above difficulties in the display panel wherein a true black color can be rendered in a high contrast ratio panel.
Object of the present invention:
With a view to the above mentioned problems associated with the light absorbing material and the formation procedure, the primary object of the present invention is to provide a light absorbing material which has excellent light absorbing property with a good vividness.
The present invention provides a true black thick film in the inter pixel gap of the display array of the pixels of the PDP comprising a true black coloring rendering agent. This light absorbing material is highly durable and does not suffer any degradation while in processing and operation of the flat panel display operating lifetime since it does not contain any degradable organic material.
The present invention also provides a set of suitable amorphous matrix composition which is suitable to work as a host to the light absorbing black agent.
The present invention also provides a process for manufacturing of such a high quality light absorbing material.
The present invention also provides a PDP using such a light absorbing material.
According to yet another aspect of the present invention a display panel is made with highly improved dark room contrast ratio compatible with 1,00,000:1. The black color rendered by the light absorbing material is compared with the RAL color system and validated in the display device.
According to yet another aspect of the present invention the light absorbing additive in the glass matrix material is added to an extent of 50% of the glass matrix.
Brief description of the drawings
Fig.1 is a perspective view illustrating the basic structure of a PDP relating to the present invention.
Fig.2 is a plan view of the display showing the light absorbing material arrays.
Fig3. is a flow chart describing the process for preparation of the precursor glass for the light absorbing material.
Fig4. is a flow chart describing the process for preparation of the light absorbing additive.
Fig5. is a flow chart describing the process for preparation of the light absorbing glass powder and paste.
Fig6. is schematic of the absorption spectra of the black matrix material of the prior art.
Detailed description of drawings
As in figure 1, the front glass substrate (1) and back glass plate (2) are shown. In the front glass substrate (1), display electrodes are made of transparent ITO electrodes (3). To reduce the resistance of the display electrode, silver bus electrodes (4) are made over the ITO electrodes. The display electrode is covered with a transparent dielectric layer (5) to limit the discharge current. Then the MgO layer (6) is deposited over the transparent dielectric layer (5) to protect the transparent dielectric layer from possible sputtering. On the back glass substrate (2), a plurality of address electrodes (7) are formed with one address electrode (7) is formed below each sub-pixel of Red, Green and Blue. The address electrodes (7) are covered with a white dielectric layer (8) to limit the discharge current and for reflection of the light formed in the pixels. The straight channel barrier ribs (9) are formed over the white dielectric layer. The R (10a), G (10b), B (10c) phosphor layers are formed in the barrier rib (9) channel spaces. The two plates are sealed with the help of sealing frit and the panel is baked at 350C. After the panels cool down to the room temperature, plasma gas mixture of Ne-Xe is filled in at a particular pressure.
As in figure 2, the front glass contains an array of plurality of electrodes (1,5) formed on the transparent ITO electrodes (2) in the discharge spaces. The discharge starts at the discharge gap (4) and spreads. The parallel set of display electrodes forming the discharge display space are separated by a non discharge space (3) which is covered by the black matrix (3).
As in figure 3, the raw materials for the precursor glass materials are mixed in the proportion and melted in the platinum crucible. When the sufficient melting occurred, the liquid mass was quenched in cold air blast on which the mass gets solid. The precursor glass thus formed was then ground dry and sieved through the designated mesh #B.S.S 500. The dry grinding process was so optimized that the 98% of the glass powders was less than 8 micron in size.
As in figure 4, the coloring oxides were mixed according to the required composition and were mixed in a planetary mill in presence of sufficient quantity of liquid methyl alcohol medium. The grinding balls in the mill help the difference in the particle size of the oxide materials even out. This mixing grinding phase was so optimized that the average size of the particles remains below 10 micron. After the process is over, the resulting slurry was dried in air drier at 120C to remove the organic alcohol material.
As in figure 5, the precursor powder and the mixed coloring oxide materials obtained earlier were mixed in such a way that the ultimate oxide composition in the final light absorbing glass material was obtained. The powders were mixed in a liquid medium in a planetary mill as was done in the previous case and dried. The mass of such compositions was put in ceramic crucibles and was fired at 600-700C depending on the requirement and the mass after cooling was taken out by breaking the crucibles. The resulting mass was dry ground to the required particle size.
The light absorbing glass powder as obtained in the previous step was then mixed with 27-35% of organic vehicle mixed with ethyl cellulose. The exact quantity of the vehicle was determined by the workability of the paste thus made. The mix of the powder and vehicle was homogenized in a Thinky Planetary mixer for 2 mins. The paste thus made was dispersed in a ceramic three roller Mill and the resultant paste was tested in Hegmann Gauge for dispersion. The paste thus made was de-bubbled on a vibrator table and was ready for final use.
Description of the preferred embodiments
As a method of forming an light absorbing dark colored insulating glass composition as mentioned in the present invention, it is commion to employ a method wherein the frit glass is pulverized to a very fine powder, the fine powders were then mixed to an organic vehicle whereby a paste was prepared to be further used for screen printing followed by drying, firing for removal of the solvents and characterized for evaluating different properties. However, the method is not particularly limited to such a method, so long it is capable of forming a thick film layer which accomplices the object of the present invention.
Now the present invention will be described in further detail with reference to examples. However, it should be understood that the present invention is by no means restricted by such specific Examples.
EXAMPLES 1to21
Various oxides were added to prepare the glass precursor matrix in the system Si02-AI203-B203-ZnO-PbO. The raw materials are dry mixed and placed in a platinum crucible. The mass was fired in the temperature range of 1300-1350C depending on the melt-ability and the final melt viscosity. The temperatures were so chosen that the glass melt after preparation should be easily transferable to quenching container. This glass melt after air quenching, was ground to desired particle size of average 45 micron. Table 1 contains the Oxide composition of the precursor glass matrix powder.
Further, definite quantity of powdered oxide materials like Cobalt Oxide. Chromic Oxide, Ferric Oxide, Manganese di-oxide and Nickel Oxide were taken in a planetary ceramic rapid mill. The mixture of the oxides was made to mix homogenously inside the mill in presence of sufficient quantity of 50% methyl alcohol and 50% deionised water solution. The mill was run for 2minutes and the slurry inside was transferred to a rectangular open top shallow container and was placed inside electrically heated drier at 120C for an hour to dry out the liquid phase in it. After the oxide mixture was dried properly, it was de agglomerated and passed through SS laboratory sieve B.S.S #240 and the pass out material was taken for further experimentation.
Table 2 contains the oxide composition of the coloring additives made during the experimentation.
In the next step the glass mentioned in the Table 1 and the coloring additives mentioned in the Table 2 were mixed in definite proportions as mentioned in Table 3 and Table 4. This mixture was placed inside a planetary mill with conditions same as described in the coloring-oxides-mix preparation procedure. All the compositions of the glass-oxides mixture was then placed in ceramic crucibles and fired individually to 600-700C for 1 hour at peak temperature as was found required to complete the melting-sintering process. These masses were taken out of the furnace, cooled to room temperature. The ceramic crucible was then broken carefully and the inside mass was separated. These individually obtained masses were ground to desired level of fineness in a dry grinding mill. For the further process of paste preparation the particle size at this level was kept at 5-10 micron.
100g of these resultant powders were kneaded with 25g of organic Ethyl Cellulose vehicle having 7% of ethyl cellulose di-ethylene glycol mono-butyl ether acetate to obtain a paste like ink.
Then, soda-lime-silica glass substrates and PD200 glass substrates having a size of 100mm X 100mm and a thickness of 2.8mm and 1.8mm were prepared. The above mentioned paste was then uniformly screen printed using a Stainless Steel SS 304 # 240 screen mesh with opening of greater than 55 micron on an area of 75mm x 75mm of the glass substrate. The printed plates were then dried at 120C. Subsequent to that the dried plates were heated to 580C at a constant temperature ramp up rate of lOC/min with a soaking of 30 minutes at the peak temperature. The thickness of the
thick film on the glass substrates was measured with a Nikon digital thickness gauge and was maintained at 12 to 15 micron. Further, the substrates used in the experimentation of the present invention were all having visible light (400-800 nm) transparency of greater than 82% and thus have high transparency. The compatibility of the colored glasses with the substrate glasses was checked by the match of the thermal expansion co-efficient. The mismatch of the co efficient of the thermal expansion will lead to either compressive or tensile stress on the film. Since the thick film printing thickness is 3 orders lower than the substrate thickness, the said film will not be able to deform the glass substrate.
Table 1 Wt% composition of the precursor glass

(Table Removed)
As described in the foregoing, the light absorbing colored glass ink makes a compatible match with the glass substrate and the transparency of the coated plate is less than 1% and ranges between 0.15% to 1%.
A 42" plasma display panel was fabricated by adding this light absorbing black glass material in the inter pixel zone of the front plate using a suitable process like screen printing. The process for the application of this material is not limited to screen printing only, this light absorbing material can also be added to photo- sensitive transfer medium and applied to the plasma display front panel by photo process. The Bright Room Contrast Ratio was measured.
Further, the color of the thick film corresponds to the RAL classic color range of 9004, 9005, 9011 & 9012 and thus it is useful for flat panel displays specially the plasma display panels.

What is claimed is:
A homogenous colored glass composition consisting essentially of , as represented by weight % of oxides, from 0 to 10% of SiO2, from 0 to 5.7% of AI2O3, from 7.7 to 17.4 % of B2O3, from 4.4 to 16.7% of ZnO, from 35.6 to 50.8% of PbO, from 3.8 to 4.4% of ZrO2, from 6.6 to 10.7% of Co3O4, from 1.2 to 3.0% of Cr2O3, from 6.4 to 11.1% of Fe2O3, from 2.2 to 4.7% of MnO2 and from 2.9 to 5.0% of Ni2O3.
2. The glass composition as claimed in the claim 1, which has a matrix glass forming oxide composition excluding the coloring additives, as represented by weight % of oxides, from 0 to 16.4% of SiO2, from 0 to 8.1 % of AI2O3, from 7.6 to 27.8 % of B2O3, from 3.2 to 27.3% of ZnO, from 51.2 to 73.2% of PbO.
3. The glass composition as claimed in the claim 1, contains coloring additives excluding the precursor glass matrix, as represented by weight % of oxides, from 10.5 to 16.7% of Zr02, from 23.9 to 29.5% of Co304, from 4.3 to 8.9% of Cr2O3, from 25.2 to 33.1% of Fe2O3, from 8.7 to 14.2% of MnO2 and from 10.7 to 16.4% of Ni2O3.
4. The glass composition as claimed in the claim 1, which has a matrix glass forming oxide composition excluding the coloring additives, as represented by Mol % of oxides, from 0 to 34.2% of SiO2, from 0 to 10.5 % of AI2O3, from 15.6 to 50.8 % of B2O3, from 5.3 to 40.81% of ZnO, from 24.6 to 45% of PbO.
5. The glass composition as claimed in the claim 1, contains coloring additives excluding the precursor glass matrix, as represented by Mol % of oxides, from 12.9 to 21% of ZrO2, from 14.9to 19.2% of Co3O4, from 4.25 to 8.9% of Cr2O3, from 23.8 to 31.8% of Fe2O3, from 15.5 to 24% of MnO2 and from 10.1 to 15.5% of Ni203.
6. The glass composition as claimed in the claim 1, as represented by mol wt % of oxides, from 0 to 25% of SiO2, from 0 to 10% of AI2O3, from 5 to 40 % of B2O3, from 3 to 30% of ZnO, from 15 to 40% of PbO, from 3 to 5% of Zr02, from 3 to 7% of Co3O4, from 1 to 3% of Cr203, from 5 to 11% of Fe203, from 3 to 8% of Mn02 and from 2 to 5% of Ni2O3.
7. The glass composition according to the claim 1, which has average thermal expansion co efficient of from 80x10-7 to 88x10-7/C within a range of from 50C to 350C.
8. The oxide glass composition according to claim 1, which is used for light absorbing black layers covering the inter-pixel gap of the display pixel arrays between a pair of sustain and scan electrodes of plasma display panel.
9. The substrate of claim 5, wherein the light absorbing glass has a working temperature at most 600C.